This invention relates to a circuit arrangement for operating a lamp comprising
input terminals for connection to a supply voltage source, PA1 a DC-AC-converter coupled to said input terminals for generating a high frequency signal, PA1 a piezo-electric transformer having a primary side that is coupled to the DC-AC-converter and a secondary side that is coupled to terminals for lamp connection, and PA1 means for adjusting the light output of the lamp. PA1 a light source provided with a vessel which is closed in a gastight manner and transmissive for visible radiation, PA1 a housing connected to the light source and provided with a lamp cap, PA1 ballast means electrically connected to the light source for operating the light source and positioned at least partly in a space surrounded by the housing.
The invention also relates to a compact lamp and a liquid crystal display.
A circuit arrangement as mentioned in the opening paragraph is known from DE-OS 2611135. In the known circuit arrangement the DC-AC-converter comprises a self-oscillating circuit so that the signal that is present at the primary side of the piezotransformer during operation of the circuit arrangement is substantially sinusoidal. Dimming or adjusting the light output of the lamp is realized by adjusting the amplitude of the voltage that is present at the primary side of the piezo-electric transformer. This can be done for instance by coupling a transistor or potentiometer between the DC-AC-converter and the primary side of the piezo-electric transformer and adjusting the conductivity of the transistor or the resistance of the potentiometer respectively. A disadvantage of this way of dimming is that when the light output is adjusted to a relatively low level, a relatively high amount of power is dissipated in the transistor or the potentiometer respectively. Therefore, the efficacy of the known circuit arrangement is relatively low when the lamp is dimmed.